1. Field of the Invention
This invention relates to sealing devices, particularly sealing devices for elongate heating cables.
2. Introduction to the Invention
It is often necessary to seal the end of an elongate cable such as a power cord or heating cable in order to provide electrical insulation, environmental protection, and/or mechanical shielding. A number of methods have been used to provide such a seal, including heat-recoverable tubing, end-caps, or boots; molded adhesive-filled boots; enclosures with gaskets or grommet seals; and wrapped tape. Many of these methods are craft-sensitive, difficult to install, require special tools, or are limited to a particular size or shape cable. In addition, some of the techniques require multiple parts which are difficult to manufacture and assemble.
Elongate heating cables are one type of cable which often requires an end seal. Such heating cables are known for use in the freeze protection and temperature maintenance of pipes. Particularly useful elongate heating cables comprise (a) first and second elongate electrodes, (b) a plurality of resistive heating elements connected in parallel between said electrodes, e.g. a continuous strip of a conductive polymer in which the electrodes are embedded or which is wrapped around the electrodes, and (c) an insulating jacket which surrounds the electrodes and heating elements. In addition, the heating cable often also comprises a metallic grounding layer, in the form of a braid or a tape, surrounding the insulating jacket, which serves to electrically ground the heating cable and provides abrasion resistance. The heating cable may be cut to the appropriate length for use in each application, and the ends must then be protected.
End caps and sealing devices for protecting the cut ends are known. These sealing devices not only seal the ends, but also prevent exposure to moisture and grip the heater in order to prevent pullout of the heater from the sealing device during use. The device disclosed in U.S. Pat. No. 4,751,350 (Eaton) comprises a cap in which a plurality of resilient projections positioned at specific angles with the cap are used to grip an electrical cable. The cap provides a particularly effective seal when it is filled with a gel or other conformable material which effectively contacts the inserted cable. The device disclosed in U.S. Pat. No. 4,877,943 (Oiwa) comprises a cap in which projections of graduated length positioned at specific angles are used to grip both the insulating jacket and the metallic braid of a heating cable in order to prevent the braid from slipping with respect to the insulating jacket which would decrease the pullout force, i.e. the amount of force required to remove the heating cable from the sealing device. The disclosures of both of these documents are incorporated herein by reference.
One problem with the conventional sealing devices disclosed by U.S. Pat. Nos. 4,751,350 and 4,877,943 is that relatively high insertion force, e.g. 3.6 to 4.5 kg (8 to 10 pounds), is required to insert the cable into the device. This is because it is necessary to deflect the projections inside the device sufficiently to engage the cable. In addition, a material, usually a plastic, that will allow such deflection often will not be sufficiently tough to provide requisite impact resistance. Thus, if the device is to be economically manufactured, for example, by molding, a compromise on the material used for the housing and the projections must be made. During impact testing, the resulting sealing device may experience flexing and deformation of the projections, decreasing the pullout force. Furthermore, despite the fact that sealing devices with projections of graduated length are known, it is often necessary to have devices of different sizes in order to accommodate and maintain adequate pullout force for cables of different thicknesses and/or widths.